1. Field of the Invention
The present invention relates to an electrode for discharge tube, and a discharge tube using it.
2. Related Background Art
Discharge tubes are commonly used as light sources for illumination and instrumentation. The discharge tubes are light sources in which a cathode and an anode are included opposite to each other in a discharge gas atmosphere and in which arc discharge is induced between the cathode and the anode. Such discharge tubes are provided, for example, with the electrode as disclosed in Japanese Utility Model No. H04-3388. This electrode is one having such structure that the tip of a refractory metal rod is covered by an impregnated electrode of a cap shape obtained by impregnating a porous refractory metal with an electron-emissible substrate. When the discharge tube is constructed using the electrode of the porous refractory metal impregnated with the electron-emissible substance as in the case of the above electrode, the discharge tube becomes able to emit electrons readily and suffers less damage at the tip.
However, the above discharge tube, particularly, the above electrode used in the discharge tube, had the following problem. Since the above electrode uses a rodlike member, i.e., the refractory metal rod as a base section of the electrode, the contact area is small between the impregnated electrode as a main body of the electrode and the refractory metal rod, so that heat transfer efficiency is considerably low between the impregnated electrode and the refractory metal. Therefore, the heat generated in the impregnated electrode is not dissipated efficiently.
In order to solve this problem, it can be considered to employ an electrode with increased heat radiation efficiency in such structure that the base section of the electrode is provided with an end face having a projection and that the projection is placed in an insert hole of the main body of the electrode, so as to increase the contact area between the base section and the main body of the electrode.
Even in the structure of the above electrode, however, there is a small clearance between the base section and the main body of the electrode and the heat radiation efficiency is not satisfactory. With existence of such a clearance, the electron-emitting (or -emissible) substance remaining in this clearance will evaporate with arise in temperature during operation of the discharge tube to be deposited on the wall surface of the discharge tube. As a result, the discharge tube will decrease its quantity of output light, and the life of the discharge tube will be shortened.
It is, therefore, an object of the present invention to solve the above problem and provide a discharge tube with high heat radiation efficiency and with a long life and a discharge tube electrode used therein.
In order to accomplish the above object, an electrode for discharge tube according to the present invention is a discharge tube electrode used in a discharge tube in which a cathode and an anode are included opposite to each other in a discharge gas atmosphere and in which arc discharge is induced between the cathode and the anode, the electrode comprising a base section made of a refractory metal and having an end face provided with a projection, and a main body made of a refractory metal containing an electron-emissible substance, having a cusp at one end thereof, and having an end face provided with an insert hole to accommodate the projection of the base section, at another end, wherein a clearance between the end face of the base section and the end face of the main body is sealed with a brazing filler metal.
When the projection of the base section is fitted in the insert hole of the main body, the end face of the base section provided with the projection comes to face the end face of the main body provided with the insert hole. Since the clearance between the end face of the base section and the end face of the main body is sealed with the brazing filler metal, the heat transfer efficiency is increased between the main body and the base section. Since the clearance between the end face of the base section and the end face of the main body is sealed with the brazing filler metal, the electron-emissible substance is prevented from entering the clearance from the outside, and even if the electron-emissible substance bleeds out of the main body into the clearance the electron-emissible substance will be prevented from being emitted from the clearance to the outside.
In the discharge tube electrode of the present invention, the brazing filler metal may be filled in the clearance.
When the clearance is filled with the brazing filler metal, the heat transfer efficiency is further increased between the main body and the base section through the brazing filler metal.
In the discharge tube electrode of the present invention, the end face of the base section may be larger than the end face of the main body.
When the end face of the base section is greater than the end face of the main body, the heat radiation efficiency of the main body is increased.
In the discharge tube electrode of the present invention, the brazing filler metal may be provided so as to extend from the clearance to a side face of the main body.
When the brazing filler metal is provided so as to extend from the clearance to the side face of the main body, the electron-emissible substance bleeding out of the side face of the main body is prevented from being emitted to the outside.
In the discharge tube electrode of the present invention, the main body may be comprised of an impregnated metal made by impregnating a porous refractory metal with an electron-emissible substance.
When the main body is comprised of the impregnated metal obtained by impregnating the porous refractory metal with the electron-emissible substance, the electron-emissible substance becomes uniformly included in the main body, so as to enhance uniformity of output light. For making the main body contain the electron-emissible substance by impregnation, the main body is normally impregnated with the electron-emissible substance after the projection of the base section is inserted into the insert hole of the main body. Since the clearance between the end face of the base section and the end face of the main body is sealed with the brazing filler metal, the electron-emissible substance is also prevented from entering the clearance during the impregnation with the electron-emissible substance.
In the discharge tube electrode of the present invention, the brazing filler metal may be a material having a melting point lower than those of the main body and the base section and higher than an impregnation temperature for the impregnation of the main body with the electron-emissible substance.
When the brazing filler metal is the material having the melting point lower than those of the main body and the base section, the shapes of the main body and the base section are maintained even during the sealing operation of the clearance by heating to melt the brazing filler metal. Since the brazing filler metal is the material having the melting point higher than the impregnation temperature, the brazing filler metal is prevented from evaporating or deforming during the impregnation.
In the discharge tube electrode of the present invention, the brazing filler metal may be a molybdenum (Mo)-ruthenium (Ru) brazing filler metal.
In the discharge tube electrode of the present invention, the electron-emissible substance may comprise a simple substance or an oxide of an alkaline earth metal.
When the electron-emissible substance is a simple substance or an oxide of an alkaline earth metal, it becomes feasible to effectively decrease the work function of the main body.
The discharge tube electrode of the present invention may further comprise a coating of a refractory metal for covering the surface of the main body while exposing the tip of the cusp of the main body.
With provision of such a coating, the electron-emissible substance bleeding out of the side face of the main body can be prevented more effectively from evaporating to the outside.
In order to accomplish the above object, a discharge tube of the present invention is a discharge tube in which a cathode and an anode are included opposite to each other in a discharge gas atmosphere and in which arc discharge is induced between the cathode and the anode, wherein at least one of the cathode and the anode is either of the discharge tube electrodes described above.
When the discharge tube is constructed using either of the above electrodes, the electron-emissible substance is prevented from going from the outside into the clearance between the end face of the base section and the end face of the main body, and even if the electron-emissible substrate bleeds out of the main body into the clearance the electron-emissible substance will be prevented from being emitted from the clearance to the outside.